The effect of pH on calcium and phosphorus distribution between micellar and serum phase after enrichment of skim milk with calcium d-lactobionate

International audienceSpeciation is important for good bioavailability of calcium from food and beverages. Effective calcium enrichment of dairy products for prevention of osteoporosis accordingly depends on the calcium compound added. Calcium d-lactobionate (50, 30 or 3 mM) was added to reconstituted skim milk with pH adjusted to 6.6, 6.0, 5.7 and 5.4 at 25 °C in order to investigate the distribution of calcium and phosphorus between micellar and serum phase and the speciation of calcium in the serum phase for enriched milk. The calcium from added calcium d-lactobionate was found to distribute between the micellar and serum phase at pH 6.6–6.0, while at pH 5.7–5.4, the added calcium remained in the serum phase. The concentration of phosphorus after enrichment was increased in the micellar phase and decreased in the serum phase at pH 6.6–6.0, while at pH 5.7–5.4, phosphorus remained constant in both phases. A decrease of total protein content and caseins in milk serum phase was observed after milk enrichment at all pH values as documented by protein determination and sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE). Moreover, concentration of α-lactalbumin remained unchanged, while concentration of β-lactoglobulin decreased after addition of calcium d-lactobionate at all investigated pH values. Bioavailability of calcium from enriched dairy products depends on transfer to the micelles, and pH control is accordingly concluded to be critical for the use of calcium d-lactobionate for enrichment. The pH value needs to be taken into account when new calcium-enriched dairy products are designed to fulfil the primary role of having a product with high nutritional value and good calcium bioavailability

Calcium induced skim-milk gelation during heating as affected by pH

International audienceAbstractMilk gels (acid or rennet) are used by dairy industry to produce dairy products such as yogurt and cheese. Enrichment of milk with calcium salts and heat treatment are known to produce “calcium-milk coagulum” as a new type of milk gels, due to reduction of milk protein charges through calcium binding. The combination of heat treatment and calcium addition to milk results in gel structures, but the effect of calcium addition and pH adjustment during heating of milk is still unclear. The role of added calcium and decreasing pH were investigated by addition of calcium chloride (30 mM) to reconstituted skim milk followed by pH adjustment by hydrochloric acid and sodium hydroxide (4.6 < pH < 6.6 investigated), followed by heating at 90 °C for 10 min and overnight storage at 22 °C. In parallel, samples with no addition of calcium chloride were produced under the same conditions. The time and temperature to reach the gelation point, as detected by dynamic measurements of storage modulus (G′), were decreasing as pH decreased without addition of calcium, while calcium addition made gelation time and temperature independent of pH except for pH 4.6. Heat treatment combined with calcium addition was found, using confocal laser microscopy, to provide a fine and dense gel structure for skim milk with higher pH, while at pH lower than 5.6, the gel structure was similar to the structure of acid-induced gels. The last observation helps to establish a pH limit for production of calcium gels